In order to improve the degradation of communication quality due to a sharp recent increase in mobile traffic and to provide faster high-speed communication, the 3GPP Long Term Evolution (LTE) has specified the Carrier Aggregation (CA) function that enables a radio terminal (User Equipment (UE)) to communicate with a radio base station (eNode B (eNB)) by using a plurality of cells. Note that cells that a UE can use in CA are limited to a plurality of cells of a single eNB (i.e., a plurality of cells operated or managed by a single eNB). The cells used by the UE in CA are categorized into a primary cell (PCell) that has already been used as a serving cell when CA is started and a secondary cell(s) (SCell(s)) that is used additionally or in a dependent manner. In the PCell, when a radio connection is (re-) established (Radio Resource Control (RRC) Connection Establishment, RRC Connection Re-establishment), Non Access Stratum (NAS) mobility information and security information (security input) are transmitted (see section 7.5 of Non-Patent Literature 1).
From a functional point of view, the introduction of CA has enabled high-speed communication. In practical usage, however, it is considered that it would be difficult to address the issue of a further increase in mobile traffic in the future due to limitations (shortage) of frequencies allocated to each operator. Accordingly, in the 3GPP standardization process, discussions on Unlicensed LTE that executes LTE with the use of an unlicensed frequency (unlicensed frequency band, unlicensed spectrum) have been started (Non-Patent Literature 2 and 3). Unlicensed LTE is also referred to as LTE-U or U-LTE and is hereinafter referred to as LTE-U.
As methods for achieving LTE-U, two methods, i.e., Licensed Assisted Access (LAA) in which the eNB performs communication with the UE on the unlicensed frequency in association with the licensed frequency (e.g., as SCell of CA) and Standalone (SA) in which the eNB performs communication with the UE only on the unlicensed frequency, are considered. The unlicensed frequency is, for example, 5 GHz band, which is also used by other systems such as radar systems and wireless LAN (WLAN or also referred to as WiFi). Therefore, with regard to the SA scheme in which communication is performed only on the unlicensed frequency, it would be difficult to implement sophisticated controls specified for LTE and thus the more feasible LAA scheme (also referred to as LA-LTE) has mainly been discussed. In the following description, LTE-U by the LAA scheme, in which CA using the licensed frequency and the unlicensed frequency is performed, will be mainly explained. The licensed frequency means a dedicated frequency allocated to a specific operator. The unlicensed frequency means a frequency that is not allocated to a specific operator or a shared frequency allocated to a plurality of operators. In the latter case, this frequency may be referred to as a licensed shared frequency, not an unlicensed frequency, and communication using this frequency is also referred to as a Licensed Shared Access (LSA). In the following description, frequencies other than licensed frequencies licensed only to any specific operators are collectively referred to as an unlicensed frequency.
LTE-U by the LAA scheme is basically executed in accordance with the sequence shown in FIG. 15. In this example, it is assumed that the eNB performs data transmission (or reception) with a UE #1 in a Cell #1, which is the licensed frequency, and a cell #2, which is the unlicensed frequency. First, a radio connection is established between the eNB and the UE #1 in the Cell #1 (RRC Connection Establishment, S901) and a bearer is established between a core network (Evolved Packet Core: EPC) and the UE #1 (not shown). That is, the Cell #1 becomes the PCell for the UE #1. When there is downlink (DL) user data (also referred to as User Plane (UP) data) to be transmitted to the UE #1 or when there is uplink (UL) user data that the UE #1 wants to transmit, the eNB transmits or receives this user data in the Cell #1 (DL (or UL) UP data transmission, S902).
Next, when the eNB determines that it is efficient for the UE #1 to transmit and receive user data in the Cell #2 at some point (Trigger LTE-U for UE #1, S903), the eNB transmits, to the UE #1 in the Cell #1, control information about radio resource configuration for the Cell #2 (Radio Resource Configuration for Cell #2, S904). This control information corresponds to a RadioResourceConfigDedicated Information Element (IE) and a RadioResourceConfigCommon IE transmitted in an RRC Connection Reconfiguration message of the LTE (Non-Patent Literature 4). The Cell #2 hereby becomes the SCell for the UE #1. When the user data is transmitted in the downlink, the eNB performs sensing in the Cell #2 to determine whether the Cell #2 is available (Perform channel sensing, S905). Upon determining that the Cell #2 is available, the eNB transmits or receives user data to or from the UE #1 (DL (or UL) UP data transmission, S906). As described above, by the use of the unlicensed frequency, it is expected that the throughput will be further improved or the cell capacity will be increased.
The aforementioned sensing is referred to as Listen Before Talk (LBT) (Non-Patent Literature 2), which determines whether LTE-U by another operator or communication of another radio system (e.g., WLAN) is performed nearby on the target unlicensed frequency. The aforementioned sensing corresponds to, for example, Channel Availability Check (CAC) for radar systems and Clear Channel Assessment (CCA) executed by a WLAN Access Point (AP) (Patent Literature 1).